Ultrafast creation of a light-induced semimetallic state in strongly excited 1T-TiSe2

Screening, a ubiquitous phenomenon associated with the shielding of electric fields by surrounding charges, has been widely adopted as a means to modify a material’s properties. While most studies have relied on static changes of screening through doping or gating thus far, here we demonstrate that screening can also drive the onset of distinct quantum states on the ultrafast timescale. By using time- and angle-resolved photoemission spectroscopy, we show that intense optical excitation can drive 1T-TiSe2, a prototypical charge density wave material, almost instantly from a gapped into a semimetallic state. By systematically comparing changes in band structure over time and excitation strength with theoretical calculations, we find that the appearance of this state is likely caused by a dramatic reduction of the screening length. In summary, this work showcases how optical excitation enables the screening-driven design of a nonequilibrium semimetallic phase in TiSe2, possibly providing a general pathway into highly screened phases in other strongly correlated materials.

temperature with XUV light, shown in Figure 1b.The sample on which this data is taken has a slightly higher amount of Ti self-doping and thus shows a more pronounced conduction band (66).All ARPES data in this paper were analyzed using pyARPES, an open-source python-based analysis framework (67).Momentum distribution curves (MDCs) have been smoothed with the Gaussian method over a window of 0.0075 Å -1 (0.005 Å -1 in Figure 1).The ARPES image plots shown in Figure 1b-e have been smoothed by 10 meV and 0.0075 Å -1 .Furthermore, for enhanced clarity we subtract an exponential background above and a constant background below the Fermi level for all image plots of excited spectra.As shown in Figure S1, the linear dispersing states above the Fermi level are already clearly visible in the raw data.
To correct for rigid band shifts due to for instance space charge effects we aligned all spectra in Figure 2 and 3 of the main text in energy to the Se 4p-1 position at the side of the band (-0.12Å -1 ). Figure S2b and c show EDCs taken for different excitation densities with and without aligning, respectively.The region where the EDCs are extracted is indicated by the white line in panel a.The Se 4p-1 position is obtained by fitting the EDCs with two Voigt peaks on a linear background.

Supplementary Note 2: MDC Fitting
Figure S3 shows schematically fitted MDCs at high and low binding energies.For low binding energy the MDCs are fitted with two Lorentzian peaks with a linear background, corresponding to the two spin-orbit split selenium bands, whereas close and above the Fermi level only one peak corresponding to the Se 4p-1 band is necessary.

Supplementary Note 3: Band Overlap in the Semimetallic Phase
The light induced semimetallic state shows a pronounced indirect band overlap between valence band at A point and the conduction band at L point.Unfortunately the XUV probe energy of 22.3 eV is not high enough to fully map the A and L point simultaneously.To estimate the overlap ∆, in Figure S4 we fit the dispersion of the Ti 3d conduction band extracted by fitting EDCs with single Gaussian peaks on a linear background between -0.8 to -0.4Å -1 (orange markers) with a parabolic function and extrapolate it to the L point (solid orange line).The maximum of the valence band at A point is obtained by fitting the EDC again with a single Gaussian peak on a linear background and is indicated by the black line, the dispersion of the valence band is schematically shown as a guide to the eye by the solid red line.In doing so one obtains an indirect band overlap ∆ between conduction band minimum at L point and valence band maximum at A point on the order of ∼350 meV.

Supplementary Note 4: Polarization Dependence
Figure S5 shows a systematic comparison between different XUV probe polarizations.In the spectra taken with p-polarization the spin-orbit splitting is not well resolved and consequently the light-induced semimetallic state is barely visible.Supplementary Note 5: Equilibrium Band Structure at High Temperatures Figure S6 shows data taken at 300 (panel a) and 400 K (panel b) at the microARPES endstation at beamline 7.0.2(MAESTRO) at the Advanced Light Source.Samples were measured using a Scienta R4000 Hemispherical Analyzer using circular polarized light with a photon energy of 119 eV.

Supplementary Note 6: Computational Details
First-principles calculations were performed within the density functional theory (DFT) with ultrasoft pseudopotentials (68), as implemented in the Quantum ESPRESSO package (69,70), while employing a kinetic energy cutoff of 52 Ry for the plane-wave expansion of the Kohn-Sham wavefunctions and 575 Ry for the density.For the exchange-correlation potential we have adopted the DFT+U approximation using as semilocal DFT exchange and correlation kernel the generalized gradient approximation in the Perdew, Burke and Ernzerhof (PBE) (71) formulation.An on-site Hubbard U parameter equal to 3.5 eV was added to the PBE Hamiltonian, in the rotationally invariant scheme of Liechtenstein et al. (54).The Hubbard U value was chosen in order to best reproduce the measured electronic structure.While the local correlations included via the Hubbard U do not correctly reproduce TiSe 2 's dynamical instability (23), they still allow an excellent description of its electronic structure both for the CDW and the normal state (23) as long as the experimental structure is used and with a much lower computational cost with respect to hybrid approaches (21,72).As our calculations are aimed to the interpretation of the experimental electronic structure, we employed the experimental structural parameters both for the normal and charge-density wave phase (33,73).A 24×24×12 Monkhorst-Pack wave-vector grid (74) has been adopted for the integration of the Brillouin zone of the normal state unit cell, and the sampling was adapted consistently in the supercell calculations in order to maintain the same k-point density.Spin-orbit coupling was included in the calculations.Due to the semimetallic nature of the compound, a smearing approach (Methfessel-Paxton smearing (75) of 0.01 Ry) has been used to converge the self-consistent calculations.
Further calculations were performed employing a reduced value for the Hubbard U (2.5 eV, ≈28.6% reduction of the original value) and a reduced value for the charge-density wave distortion (0% and 50% of the initial CDW amplitude) in order to simulate the screening effects of the photoexcited carriers and the partial and total melting of the CDW phase.The theoretical calculations are performed on a 2×2×2 supercell and then unfolded to the primitive cell of TiSe 2 via the band unfolding method (76) as implemented in the BandsUP software (77).
By comparison with literature (24,34,35) we estimate that our XUV beam probes the k z plane around ∼ -0.33 Å -1 .However, due to the symmetry properties of the material there exists an indeterminacy regarding the sign of k z when comparing it with the calculations, depending on which z-axis orientation is chosen.Thus, while we show the calculated band structure for k z =-0.3Å -1 in the main text, we also report the results for k z =0.3Å -1 in Figure S7 for completeness.Comparison with Figure 4 from the main text shows that there are no qualitative differences and all key results are apparent in both k z planes.

Figure S1 :
Figure S1: Raw spectra processing.a-b) Raw ARPES spectrum at a delay of 80 fs after excitation with 200 µJ /cm 2 before (a) and after (b) background subtraction.

Figure S2 :
Figure S2: Energy Alignment a) ARPES plot at a delay of 85 fs after excitation with 40 µJ /cm 2 (spectrum identical to Figure 3a in the main text).b) EDCs extracted for different fluences.The region where the EDCs are extracted is schematically indicated by the white line in panel a. c) same EDCs as in panel b but aligned in energy.

Figure S3 :
Figure S3: MDC fitting a-b) Fitted MDC taken at -0.52 eV (a) and -0.15 eV (b) for a spectrum taken at a delay of 23 fs after excitation with 180 µJ /cm 2 .

Figure S4 :
Figure S4: Estimated band overlap.ARPES image plot after excitation with 280 µJ /cm 2 at a delay of 85 fs.Orange crosses are fits to the Ti 3d conduction band.Solid orange line is a parabolic fit to the data points which is extrapolated to L point.Solid red line schematically shows the valence band dispersion as a guide to the eye.

Figure S5 :
Figure S5: Polarization dependence of the XUV probe.ARPES image plots under equilibrium as well as after excitation with 200 (a) and 160 µJ /cm 2 (b) respectively.Spectra in panel a are taken with s-polarized, spectra in panel b with p-polarized XUV light.

Figure S6 :
Figure S6: High temperature ARPES spectra a-b) Equilibrium ARPES spectra taken at 300 (a) and 400 K (b), respectively.Spectra were taken with 119 eV.c) Extracted EDCs from A (orange) and L (green) point for 300 (solid lines) and 400 K (dashed line), respectively.

Figure S7 :
Figure S7: Calculated single-particle band structures for different lattice distortions and Hubbard U terms for k z =0.3Å -1 .